Hi there, welcome to the forum. Having stamps made has been a bit of a problem for Letterboxers ever since Tanda Stamps decided to close down. Several boxers have had stamps made elsewhere, but not sure how they have performed. One company makes an A4 sheet for £25. They are
http://www.dimensionstamps.com/.
Another one is http://www.devonstamps.co.uk/ This is run by Adrian Moore and he does give a discout for Letterboxers.